Using 555 Oscillator to get 5V output pulse.

Thread Starter

aac044210

Joined Nov 19, 2019
178
Hi:

If I have a 555 astable circuit and a 5 volt supply, should I not get an output of
approximately 5 volts ac?

Thanks
 

Thread Starter

aac044210

Joined Nov 19, 2019
178
The reason I ask is that I am trying to measure the output voltage and I get ≈ 4.95 vdc. I just
thought that it would be ac but then the polarity is always positive so I guess you can call it
dc?
 

ElectricSpidey

Joined Dec 2, 2017
1,814
Technically it would be pulsed DC.

AC is generally where the voltage goes negative and then positive around some common point usually termed as ground.

But, that is all semantics.
 

dl324

Joined Mar 30, 2015
13,141
The reason I ask is that I am trying to measure the output voltage and I get ≈ 4.95 vdc.
If you're measuring with a DVM, you're getting some average reading that depends on the output swing and the duty cycle. If you really want to see what's going on, you need an oscilloscope. They're so cheap ($20 for DSO 138 at places like Amazon and Ali Expreds) these days that there's really no excuse for choosing to remain in the dark.
 

WBahn

Joined Mar 31, 2012
26,398
Hi:

If I have a 555 astable circuit and a 5 volt supply, should I not get an output of
approximately 5 volts ac?

Thanks
Depends on what kind of 555 it is. If it's a TTL version, then the high-level output is going to be about 1.5 V to 2 V below the supply voltage. With a CMOS version it will be much closer to the supply voltage. These are under no load conditions.
 

ScottWang

Joined Aug 23, 2012
7,065
If you using like NE555 TTL type then you may just get the output pulse about 3.6V, If you want to get the output 5V from 555 that the positive Vcc powered by 5V then you have to use CMOS 555 like as LMC555, TS555, etc.,

Or add a stage PNP transistor as 2N3906 and the Base in series with an 1N4148 and 2.2K resistor to the output of NE555, and add a Rbe 4.7K between b, e pin, and e pin connects to +5V, c is the output, you can also connecting a 4.7K to Ground as a false load.
 

dl324

Joined Mar 30, 2015
13,141
I measured the output swing of some 555 timers (TI NE555 DIP from the 70's, Raytheon RC555, and TI NE555 surface mount) with unloaded outputs. The high level output was a little over 4V for all of them.
 
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